JP2790471B2 - Muting circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a signal for removing pop noise generated in an audio output signal of a demodulation circuit when switching between stereo broadcast of broadcast radio waves and bilingual (hereinafter referred to as bilingual) broadcast. The present invention relates to a muting circuit that bypasses a transmission path.

2. Description of the Related Art Conventionally, in a television audio multiplex broadcast receiver, an FM receiver, and the like, pop noise has been generated in an audio output signal of a demodulation circuit due to a circuit configuration when switching between stereo broadcast and bilingual broadcast of a broadcast radio wave. In order to remove the pop noise, a muting circuit for bypassing the signal transmission path at the time of the switching is provided.

Hereinafter, an example of the above-described conventional muting circuit will be described with reference to the drawings.

FIG. 3 shows a circuit diagram of a conventional muting circuit. In FIG. 3, 1 is a multiplex signal input terminal, 2
Is a TV audio multiplex demodulation circuit, 3 is a signal transmission path, and 4 and 5 are connected between the power supply terminal 6 and the stereo indicator terminal a and the bilingual indicator terminal b of the TV audio multiplex demodulation circuit 2 via resistors 7 and 8, respectively. In the stereo display and the bilingual display, the stereo indicator terminal a is connected to the stereo indicator terminal 9 of an FM stereo demodulation circuit (not shown), and the stereo display 4 serves both for FM stereo display and television stereo display. 10 and 11 are pulled up at both ends by resistors 12, 13, 14 and 15, respectively.Capacitors connected to indicator terminals a and b, 16 and 17 are connected at one end to indicator terminals a and b, respectively, and the other end is 18,19
This is the capacitor that was pulled down. 20,21,22,23 are diodes connected to the capacitors 10,11,16,17, respectively, 24 is a PNP type in which the base is connected to the connection point of the diodes 20 and 21 and the emitter is connected to the power supply terminal 6. In the transistor, the base is pulled up by the resistor 25. Numeral 26 denotes a muting transistor, the collector of which is connected to the signal transmission line 3 and the base of which is connected to
Connection point between the collector of diode and diode 22 and diode 23
And a portion 28 enclosed by a broken line is the control means.

The operation of the muting circuit configured as described above will be described below.

When the mode of the broadcast wave changes from monaural broadcasting to stereo broadcasting, the pilot signal detector (not shown) of the television audio multiplex demodulation circuit 2 operates to set the potential of the stereo indicator terminal a to the L level and turn the stereo display 4 on. At the same time, the television audio multiplex demodulation circuit 2 sets the demodulation mode to stereo and outputs an audio signal to the signal transmission line 3. At this time, pop noise occurs due to a problem in the circuit configuration. Therefore, the muting is performed by providing the control means 28 in a portion surrounded by a broken line, conducting the muting transistor 26 and bypassing the signal transmission line 3. ing. When the potential of the stereo indicator terminal a becomes L level, the capacitor 10
Is the voltage of the potential fluctuation (voltage of the potential difference between both ends of the resistor 13)
Is applied, the diode 20 conducts until the capacitor 10 is charged, the transistor 24 conducts, the potential at the output point c of the control means becomes H level, the transistor 26 conducts, and muting operates (fourth step). (See FIG. B). When the capacitor 10 is charged after a time determined by the time constant determined by the capacitor 10 and the resistors 14 and 25 from this state, the diode 20 becomes non-conductive, so that the transistor 24 also becomes non-conductive and muting is released. You.

Next, when the mode of the broadcast wave changes from stereo broadcasting to monaural broadcasting, when the potential at the stereo indicator terminal a becomes H level, the voltage corresponding to the variation in potential is applied to the capacitor 16 and the diode 22 conducts, and the connection point The potential of c becomes H level and muting is applied. When the capacitor 16 is charged after a time determined by the time constant determined by the capacitor 16 and the resistors 18 and 27 from this state,
Since the diode 22 becomes non-conductive, the potential at the connection point c becomes L level and muting is released.

In the case of bilingual broadcasting, a muting circuit can be similarly applied by utilizing the change in the potential of the bilingual indicator terminal b, and pop noise can be completely removed by appropriately setting the time constant.

However, in the above-described configuration, the capacitors are provided at the rising and falling of the potentials of the two indicator terminals a and b, and the charging and discharging of the capacitors are used. In some cases, a switching diode and a discharge resistor at the time of discharge are required so that other capacitors are not charged, and the circuit is complicated.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and provides a muting circuit that can eliminate pop noise generated when a mode of broadcast radio waves is switched without providing a diode for preventing charging of another capacitor when charging one capacitor.

Means for Solving the Problems To solve the above problems, a muting circuit according to the present invention comprises an OR circuit for a stereo indicator output and a bilingual indicator output of a demodulation circuit, a first capacitor having both ends pulled up, and an emitter. Is connected to a power supply terminal, a PNP transistor having a base connected to the output terminal of the OR circuit via a first capacitor, one end connected to the output terminal of the OR circuit, and the other end connected to the collector of the transistor. And a connection point between the collector of the transistor and the second capacitor is used as the output of the control means.

Operation In the present invention, the potential variation point for obtaining the muting control means is only one in the above configuration, and only one capacitor for detecting the rise and fall of the potential is required, so that the circuit can be simplified.

Embodiment Hereinafter, a muting circuit according to an embodiment of the present invention will be described with reference to the drawings. It is to be noted that components having the same functions as those in the related art will be described with the same reference numerals.

FIG. 1 is a circuit diagram of a muting circuit according to one embodiment of the present invention. Diodes 29 and 30 are connected to the stereo indicator terminal a and the bilingual indicator terminal b, respectively, 31 is a first capacitor whose both ends are pulled up by resistors 32 and 33, and 34 is an emitter having a power supply terminal 6.
A PNP transistor whose base is connected to a connection point d between the diodes 29 and 30 via a first capacitor 31. One end of the PNP transistor 35 is connected to the connection point d, and the other end is a transistor.
34 is a second capacitor connected to the collector.

The muting circuit configured as described above operates.

When the mode of the broadcast wave changes from monaural broadcasting to stereo broadcasting, the potential of the stereo indicator terminal a becomes L level, and when the mode changes from monaural broadcasting to bilingual broadcasting, the potential of the bilingual indicator terminal b becomes L level (both are not at L level at the same time). ), The potential at the connection point d becomes L level because the diode 29 or the diode 30 conducts regardless of which terminal potential becomes L level. Then, the voltage corresponding to the voltage fluctuation (the voltage across the resistor 32) is applied to the capacitor 31, so that the PN is maintained until the capacitor 31 is charged.
The base potential of the P-type transistor 34 decreases, and the transistor 34 becomes conductive. At this time, the electric charge accumulated in the capacitor 35 acts to lower the connection point e to the L level momentarily when the connection point d becomes the L level, but when the transistor 34 is conducting, the power supply side of the collector is viewed. Since the impedance is low, the potential at the connection point e immediately becomes the H level (see FIG. 2 (b)). This state depends on the capacitor 31 and the resistor
The time determined by the time constant of 33 (that is, the capacitor
(While charging to 31). When the capacitor 31 is charged, the transistor 34 is turned off.
The transistor 26 is turned on until the charged capacitor 35 discharges through the resistor 27 when the 34 is turned on.

Next, when the mode of the broadcast wave changes from stereo broadcasting to monaural broadcasting or bilingual broadcasting to monaural broadcasting, the potential at the terminal a or the terminal b becomes H level and the diode 29 or the diode 30 becomes non-conductive, so that the connection point d also becomes H. Level. Then, since the voltage corresponding to the change in potential is applied to the capacitor 35, the potential at the connection point e becomes H level until the capacitor 35 is charged for a time determined by the time constant of the capacitor 35 and the resistor 27, and the transistor 26 is turned on. Muting takes place. At this time, the charge accumulated in the capacitor 31 acts to raise the base potential of the transistor 34 above the emitter potential until the capacitor 31 discharges through the resistor 33 due to the potential of the connection point d becoming H level. There is no inconvenience if the voltage of the power supply terminal 6 is set so as not to exceed the reverse breakdown voltage.

As described above, according to this embodiment, the diodes 29 and 30 connected to the stereo indicator terminal “a” and the bilingual indicator terminal “b” respectively change the potential of either terminal even when the potential of either terminal changes. A first capacitor 31 which is pulled up by the above, a PNP transistor 34 whose emitter is connected to the power supply terminal and whose base is connected to the connection point d through the first capacitor 31, and one end is connected to the connection point d. And a second capacitor 35 having the other end connected to the collector of the transistor 34, and a control means for outputting the connection point e drives muting means for muting the audio output signal. Switching diodes are not required to eliminate the effects of other capacitors when discharging, and a simple circuit configuration is used to switch the mode of broadcast radio waves. Ppunoizu can be removed.

As described above, the present invention provides an OR circuit for stereo indicator output and bilingual indicator output, a first capacitor having both ends pulled up, an emitter connected to a power supply terminal, and a base connected via the first capacitor. A PNP transistor connected to the output terminal of the OR circuit, and a second capacitor having one end connected to the output terminal of the OR circuit and the other end connected to the collector of the transistor,
Since the muting means for muting the audio output signal is driven by the control means for outputting the connection point between the collector of the transistor and the second capacitor, the pop-up when the mode of the broadcast wave is changed with a simple circuit configuration. Noise can be removed.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a muting circuit in one embodiment of the present invention, FIG. 2 is a time chart thereof, FIG. 3 is a circuit diagram of a conventional muting circuit, and FIG. 4 is a time chart thereof. 2 ... TV audio multiplex demodulation circuit, 26,33 ... Transistor, 28,29 ... Diode.

Claims (1)

(57) [Claims] An OR circuit for outputting a stereo indicator output of an FM stereo demodulation circuit or a television audio multiplex demodulation circuit and a bilingual indicator output of a television audio multiplex demodulation circuit, a first capacitor having both ends pulled up, and an emitter. A PNP transistor connected to a power supply terminal and having a base connected to the output terminal of the OR circuit via the first capacitor, one end connected to the output terminal of the OR circuit, and the other end connected to the collector of the transistor And a second capacitor connected to the control terminal of a muting unit that bypasses a signal transmission path at a connection point between the collector of the transistor and the second capacitor.